Ferroelectric thin film element, piezoelectric actuator and liquid discharge head

Abstract

A ferroelectric thin film element comprises a substrate and an epitaxial ferroelectric thin film provided on the substrate. The thin film satisfies z / z0>1.003 and 0.997≦x / x0≦1.003, where a crystal face of said thin film parallel to a crystal face of a surface of the substrate is taken as a Z crystal face, a face spacing of the Z crystal face is taken as z, a face spacing of the Z crystal face of a material constituting the thin film in a bulk state is taken as z0, a crystal face of the thin film perpendicular to the Z crystal face is taken as an X crystal face, a face spacing of the X crystal face is taken as x and a face spacing of the X crystal face of the material constituting the thin film in a bulk state is taken as x0.

Description

BACKGROUND OF THE INVENTION[0001]1. Field of the Invention[0002]The present invention relates to a ferroelectric thin film element, and more particularly to an element in which a spontaneous polarization of a ferroelectric thin film is involved in an improvement in device characteristics, such as a non-volatile memory. The present invention also relates to a piezoelectric actuator utilizing a piezoelectric property of an epitaxial film and a liquid discharge head equipped with a piezoelectric actuator unit of a configuration including such a piezoelectric actuator.[0003]2. Description of the Related Art[0004]As a memory medium for a non-volatile memory and the like, there is recently desired a memory apparatus employing a ferroelectric thin film having a high performance (hereinafter called ferroelectric memory). For securing optimal device characteristics and reproducibility in the ferroelectric memory, the ferroelectric thin film is required to have a large spontaneous polarizatio...

AI Technical Summary

Benefits of technology

[0017]An object of the present invention is to provide a ferroelectric thin film element which not subject to deterioration of its characteristics upon application of a small stress applied in the ferroelectric thin film along a planar direction of the substrate, shows a high spontaneous polarization of the ferroelectric thin film and is suitable for thin film formation. It is effective for an element in which the spontaneous polarization of the ferroelectric thin film is involved in improving the characteristics of the ferroelectric thin film element, for example, a non-volatile memory. In heteroepitaxial growing technology, it is preferable to reduce the stress generated in the vicinity of the boundary between the substrate and the formed ferroelectric thin film and applied along the planar direction of the substrate. It is estimated that the stress generated by a misfit in the crystal lattices between the substrate and the ferroelectric thin film and applied in the planar direction of the substrate constitutes a cause of film peeling of the ferroelectric thin film, so that such film peeling can be prevented by a reduction in the aforementioned stress applied in the planar direction of the substrate.

[0018]Another object of the present invention is to provide a ferroelectric thin film element having a substrate and an epitaxial ferroelectric thin film provided on the substrate, in which the epitaxial ferroelectric thin film (1) satisfies a relation z / z0>1.003, wherein a crystal face parallel to a crystal face of a surface of the substrate, among crystal faces of the epitaxial ferroelectric thin film, is taken as a Z crystal face, a face spacing of the Z crystal face is taken as z and a face spacing of the Z crystal face of a material constituting the epitaxial ferroelectric thin film in a bulk state is taken as z0, and (2) also satisfies a relation 0.997≦x / x0≦1.003, wherein one of the crystal faces of the epitaxial ferroelectric thin film perpendicular to the Z crystal face is taken as an X crystal face, a face spacing of the X crystal face is taken as x and a face spacing of the X crystal face of the material constituting the epitaxial ferroelectric thin film in a bulk state is taken as x0. (It is understood that the term “crystal face” used herein is interchangeable with the term “crystal plane.”) According to the present invention, there can be obtained a ferroelectric thin film element free from deterioration of its characteristics, showing a high spontaneous polarization and suitable for thin film formation.

[0019]Still another object of the present invention is to provide a piezoelectric actuator having excellent characteristics, by forming an epitaxial ferroelectric film which has a reduced stress acting on the epitaxial ferroelectric film along a planar direction of the substrate, which is free from film peeling or deterioration of the characteristics of the epitaxial ferroelectric film, which enables formation of a large area, which has excellent piezoelectric characteristics and which is suitable for thin film formation. Also, another object of the present invention is to provide a liquid discharge head, particularly a liquid discharge head adapted for use in an ink jet recording apparatus, provided with a piezoelectric actuator unit of a configuration including such a piezoelectric actuator. In heteroepitaxial growing technology, it is preferable to reduce the stress generated in the vicinity of the boundary between the substrate and the formed epitaxial ferroelectric film and applied along a planar direction of the substrate. By reducing the stress generated by a misfit in lattices of the substrate and the epitaxial ferroelectric film and applied in the planar direction of the substrate, it is rendered possible to prevent film peeling of the epitaxial ferroelectric film, thereby enabling improved productivity by increasing the size of the substrate.

[0020]Still another object of the present invention is to provide a piezoelectric actuator having a substrate and an epitaxial ferroelectric film provided on the substrate, in which the epitaxial ferroelectric film (1) satisfies a relation z / z0>1.003, wherein a crystal face parallel to a crystal face of a surface of the substrate, among crystal faces of the epitaxial ferroelectric film, is taken as a Z crystal face, a face spacing of the Z crystal face is taken as z and a face spacing of the Z crystal face of a material constituting the epitaxial ferroelectric film in a bulk state is taken as z0, and (2) also satisfies a relation 0.997≦x / x0≦1.003, wherein one of the crystal faces of the epitaxial ferroelectric film perpendicular to the Z crystal face is taken as an X crystal face, a face spacing of the X crystal face is taken as x and a face spacing of the X crystal face of the material constituting the epitaxial ferroelectric film in a bulk state is taken as x0. Still another object of the present invention is to provide a liquid discharge head for discharging a liquid, utilizing the aforementioned piezoelectric actuator. According to the present invention, there can be obtained a piezoelectric actuator and a liquid discharge head which are free from film peeling and deterioration of the characteristics of the epitaxial ferroelectric film, which show excellent piezoelectric characteristics, and which are suitable for film formation and miniaturization and for forming a large area.

Problems solved by technology

However, even with an excellent ferroelectric thin film such as a PZT thin film, it is difficult to obtain satisfactory device characteristics in a ferroelectric thin film formed by a polycrystalline member, because of a distortion of physical properties at the crystal grain boundary.

These methods can cause a compression stress to be applied to the ferroelectric thin film, thereby increasing the spontaneous polarization of the ferroelectric thin film.

However the prior method of increasing the spontaneous polarization by applying a compression stress to the ferroelectric thin film, though capable of increasing the spontaneous polarization, cannot improve the deterioration in the residual polarization or the fatigue resistance of the ferroelectric thin film.

It is estimated that a stress applied in the ferroelectric thin film along a planar direction of the substrate is significantly involved in the aforementioned deterioration of the characteristics of the ferroelectric thin film, and, in case a large compression stress is applied to the ferroelectric thin film in the prior method, the stress applied along the planar direction of the substrate acts on the ferroelectric thin film, thereby further increasing the deterioration in the characteristics of the ferroelectric thin film.

However, the prior method of forming an epitaxial ferroelectric film involving a compression stress therein to increase the spontaneous polarization thereby improving the piezoelectric property, though capable of increasing the spontaneous polarization, cannot resolve drawbacks such as deterioration in the characteristics of the piezoelectric actuator over the course of repeated uses or destruction of the piezoelectric actuator induced by a leak current at the application of voltage.

It is estimated that a stress applied in the ferroelectric thin film along a planar direction of the substrate is significantly involved in the aforementioned deterioration in the characteristics and destruction of the piezoelectric actuator, and, in an epitaxial ferroelectric film prepared by the prior method and subjected to a large compression stress, the stress applied along the planar direction of the substrate acts on the ferroelectric thin film, thereby further increasing the deterioration of the durability characteristics of the piezoelectric actuator.

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